Method for aberration detection and measurement

ABSTRACT

Aberrations in an optical system can be detected and measured using a method comprised of a test target in the object plane of a projection system and imaging a photoresist film with the system. The test target comprises at least one open figure which comprises a multiple component array of phase zones, where the multiple zones are arranged within the open figure so that their response to lens aberration is interrelated and the zones respond uniquely to specific aberrations depending on their location within the figure. The method detects aberration types including coma, spherical, astigmatism, and three-point through the exposure of a photoresist material placed in the image plane of the system and the evaluation of these images.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the priority date of U. S.Provisional patent application Ser. No. 60/433,153 filed Dec. 13, 2002.

FIELD OF THE INVENTION

The present invention relates to microlithographic imaging using aprojection exposure system for fabricating semiconductor devices and thedetection of aberrations in the system

BACKGROUND OF THE INVENTION

Optical lithography has been the dominant technology for the patterningof semiconductor device features. As the size of the geometry for thesedevices continue to shrink below the ultraviolet (UV) wavelength usedfor imaging, significant demands are placed on the quality of theoptical component within the projection imaging system. The projectionsystem used for imaging of sub-wavelength features comprise a largenumber of lens elements and operate at wavelengths ranging from 436 nmto 126 nm. The level of aberration in these systems must be low enoughto allow imaging on the order of 0.30 lambda/NA, where lambda is theimaging wavelength and NA is the numerical aperature of the lens system,typically on the order of 0.40 to 0.90. This type of performance is nearthe physical limits of diffraction and aberrations must be low enough toproduce optical wavefront deformation in the projection lens pupil belowa multiple of 0.1 wavelengths, and approaching 0.01 wavelengths for themost current systems.

Lens quality can be described in terms of the ability of an opticalsystem to convert the spherical wavefront emerging from an object pointinto a spherical wavefront converging toward an image point. Eachaberration type will produce unique deviations in the wavefront withinthe lens pupil.

For a system utilizing full circular pupils, Zernike circle polynomialscan be used to represent optimally balanced classical aberrations. Anyterm in the expansion of the wave aberration function leading to acomplete set of Zernike polynomials can be represented as:

${W( {\rho,\theta} )} = {\sum\limits_{n = 0}^{\infty}\;{\sum\limits_{m = 0}^{n}\;{\sqrt{2{( {n + 1} )/( {1 + \delta_{m0}} )}}{{R_{n}^{m}(\rho)}\lbrack {{c_{n\; m}{\cos( {m\;\theta} )}} + {s_{n\; m}{\sin( {m\;\theta} )}}} \rbrack}}}}$where n and m are positive integers (n−m≧0 and even), c_(nm) and s_(nm)are aberration coefficients, and the radial polynomial R of degree n interms of the normalized radial coordinate in the pupil plane (ρ) is inMahajan's convention [V. N. Mahajan, Zernike circular polynomials andoptical aberrations of systems with circular pupils, Eng. and Lab Notes,in Opt. & Phot. News 5,8 (1994)]. Commonly, a set of 37 Zernikepolynomial coefficients is utilized to describe primary and higher orderaberration, although some applications may require additional terms.

Since any amount of aberration results in image degradation, tolerancelevels must be established for lens system, dependent on application.This results in the need to consider not only specific objectrequirements and illumination but also process requirements.Conventionally, an acceptably diffraction limited lens is one whichproduces no more than one quarter wavelength (λ/4) wavefront OPD. Formany non-lithographic lens systems, the reduced performance resultingfrom this level of aberration may be allowable. This Rayleigh λ/4 ruleis not suitable however for microlithographic applications. To establishallowable levels of aberration tolerances for a photolithographicapplication, application specific analysis must be performed.Photoresist requirements need to be considered along with processspecifications. The current needs of UV and DUV lithography require abalanced aberration level below 0.03λ OPD RMS. Future requirements maydictate sub-0.02λ performance. More important, however, may not be thefull pupil performance but instead the performance over the utilizedportion of the pupil for specific imaging situations [B. W. Smith,Variations to the influence of lens aberration invoked with PSM and OAI,Proc. SPIE 3679 (1999)]. For a good review of lithographic requirementsand tolerances, also see [D. Williamson, The Elusive Diffraction Limit,OSA Proceedings on Extreme UV Lithography (1994), 69].

Aberration metrology is critical to the production of lithographicquality lenses in order to meet these strict requirements. Additionally,it is becoming increasingly important to be able to measure and monitorlens performance in an IC fabrication environment. The lithographerneeds to understand the influences of aberration on imaging and anychanges that may occur in the aberration performance of the lens betweenlens assembly and application or over the course of using an exposuretool.

The most accurate method of measuring wavefront aberration (andsubsequently fitting coefficients of Zernike polynomials) is phasemeasurement interferometry (PMI), also known as phase shiftinginterferometry (PSI) [J. E. Greivenkamp and J. H. Bruning, Optical ShopTesting: Phase Shifting Interferometry, D. Malacara ed, (1992) 501]. PMIgenerally describes both data collection and the analysis methods thathave been highly developed for lens fabrication and assembly and used byall major lithographic lens suppliers. The concept behind PMI is that atime-varying phase shift is introduced between a reference wavefront anda test wavefront in an interferometer. At each measurement point, atime-varying signal is produced in an interferogram. The relative phasedifference between the two wavefronts at this position is encoded withinthese signals.

The accuracy of PMI methods lies in the ability to sample a wavefront. Awavefront can be sampled with a spacing of λ/n where n is the number oftimes the system is traversed by a test beam. These methods requirecareful control of turbulence and vibration. A more significantlimitation of these interferometric methods in the need for thereference and test beams to follow separated paths, making field use (orin-situ application) difficult. The lithographer is therefore restrictedto using alternative approaches to measure, predict, approximate, ormonitor lens performance and aberration.

Methods of Aberration Measurement

In addition to interferometric techniques, several methods have beendeveloped and utilized to test and/or measure optical performance.

Common-path Interferometry (and the PSPD Method)

In a conventional interferometer (such as a Twyman-Green or Mac-Zehnderused with PMI), test and reference beams must follow separate paths.This is the main difficulty with employing these methods for in-situmeasurement on a lithography tool. Common path interferometry ispossible where a reference beam travels a path through the test opticbut is done in such a way that it either does not experience aberrationor system aberrations are removed. This approach was first carried outby Burch [J. M. Burch, “Scatter Fringes of Equal Thickness”, Nature, 171(1953) 889] and has recently been applied for lithographic purposes.Workers at Lawrence Berkeley laboratories have developed Phase ShiftingPoint Diffraction (PSPD) interferometry to measure the quality of EUVoptical systems on the order of 0.02 waves RMS [P. Naulleau et al, Proc.SPIE 3331 (1998) 114]. The method utilizes a transmission grating toproduce test and reference diffraction beams. The zero diffraction orderbeam is directed through the optic being tested and experiencesaberration present within the lens pupil. A higher grating diffractionorder beam is directed toward the edge of the lens pupil and is directedthrough a small pinhole at the image side of the optic. If the pinholeis perfect, any aberration in this beam is removed. The test beam andthe reference beam are interfered and sampled for various gratingpositions to reconstruct the pupil wavefront phase. Algorithms used forthis approach are similar to those used for PMI techniques. RIT has alsoutilized this method at UV and DUV wavelengths [P. Venkataraman, B.Smith, Study of aberrations in steppers using PSPD interferometry, Proc.SPIE 4000 (2000)]. The two primary sources of error with these methodsare systematic geometric effects that arise from the geometry of thesystem (which can be compensated for if measurable) and imperfections inthe pinhole. Pinhole imperfections result in reference beam (andreference wave) error dependant on the size, shape, and positioning ofthe pinhole. There is a real limitation to the fluence that can passthough a pinhole and the fabrication capabilities required to make suchan artifact. Additionally, since interferograms must be detected beyondthe image plane, a system under test must allow access at thesepositions. Large numerical apertures will also make image capturedifficult and secondary optical relay systems may be required. AlthoughPSPD methods have a good deal of potential for accurate wavefrontmeasurement, implementation will likely be difficult withoutmodifications to stepper or scanner hardware.

Foucault Knife Edge and Wire Tests

Foucault first introduced a knife edge test, which has been modified byseveral workers and applied to many optical systems [L. M. Foucault,Ann. Obs. Imp. Paris, 5, 197 (1859)]. By blocking out part of a planewithin a lens system traversed by diffracted light, a shadow can beformed over aberrated pupil regions. The behavior of the shadow patterncan be correlated to aberration, especially spherical, defocus, coma,and field curvature. Various enhancements to this approach have provencapability at the levels needed for microlithography application butimplementation may be difficult. Mechanical slits and knife-edges (or awire in a similar test procedure) must be placed within the opticalsystem with tight tolerance over placement and parallelism.

A major limitation to these types of tests is that the test isinsensitive to small wavefront slope changes, in terms of eithermagnitude or direction. In other words, when the first or secondderivatives of the wavefront errors are small, these tests are quiteinsensitive. This is especially problematic with large apertures.

Star Tests

Probably the most basic method to test for image quality is a star test.Approaches like these examine the image of a point source and compareimage quality to an ideal. Some of the most useful comparativeinformation dates back to Taylor (H. Taylor, The Adjustment and Testingof Telescope Objectives (1891)]. Airy patterns (point spread functions)are unique for each aberration type and aberration levels to 0.05 waveshave been measured by evaluation of confined energy and intensitycontours of images. Star tests can be inherently quite qualitative and agood deal of experience is required to adequately describe an aberratedwavefront.

Star tests have been used for final rapid adjustment to balancespherical aberration in microscope objectives. By viewing images ofpinholes, an experienced user can quickly assess aberration level. Theproblem with this method is its qualitative aspect. Application tolithography may be useful for assessment purposes only. This may provedifficult, however, since diffraction limited pinhole images would bedifficult to record with any detail in photoresist.

Ronchi Tests

The Ronchi test for optical system performance has historically beenused also in a mostly qualitative way [see for instance A.Corejo-Rodriquez, Ronchi Test, Optical Shop Testing: Phase ShiftingInterferometry, D. Malacara ed, (1992) 321]. The principle of theapproach is realized when a ruling is placed near the center ofcurvature of a mirror, where the image of the grating is superimposed onthe grating itself, producing an interference pattern. This approach hasbeen used for many applications since Ronchi first introduced it in 1923[V. Ronchi, Riv. Ottica Mecc. Precis., 2, 9 (1923)]. Techniquesemploying Ronchi principles have allowed for wavefront measurement andfitting of primary and higher order aberration to a high degree ofaccuracy. These methods are limited, however, by the requirement of areflective optical system. Practical application for microlithographicpurposes is therefore also limited.

Blazed Grating Methods

Kirk and Progler have introduced a method to measure wavefrontaberration using a phase grating reticle to direct diffraction orders toparticular portions of a lens pupil [J. P. Kirk and C. J. Progler, Proc.SPIE 3679 (1999) 70]. These blazed gratings are oriented at variousangles (for example 0 to 337.5 degrees at 22.5 degree increments). Theimage of the grating is stepped through focus and imaged intophotoresist. A second blanket exposure is made, resulting in a compositeaerial image formed in a near linear response portion of the photoresistmaterial. The resulting images contain aberration information for theportion of the lens pupil sampled by the diffraction energy directed atthe blazed angle (or frequency). By using several grating angles(frequencies), both low and high order aberration terms can be fitted.Algorithms have been developed to fit this information from measuredresist images. As with many resist based evaluation methods, thecapability of this approach requires matching the images recorded inresist to simulation with various aberration type. This approach is notlimited to symmetrical aberration types because of the distribution ofgratings over a wide range of orientations. The main concern with thismethod is the ability to match high order azimuthal aberration effects.The capability of the blazed grating approach increases with increasinggrating frequencies present on the test reticle. Fabrication of thisreticle becomes challenging then as a range of etch angles must beaccommodated. Accuracy of this method has been reported to be within 12%for a single grating frequency. Improvements are possible usingadditional grating frequencies and by using lower values of partialcoherence. By using partial coherence values approaching coherentillumination, the averaging effect imparted on diffraction orders isreduced. This becomes challenging with current exposure tools that limitsigma to values above 0.3. Lower values will result in significant lossin image intensity. Careful characterization of the photoresist materialis also required for this method. Ideally, a resist should be of lowcontrast and highly absorbing (in a photochemical sense). This impliesthat the resists used for IC fabrication would not be well suited andspecial materials and modifications to processes would most likely berequired.

Aerial Image Measurement

Direct aerial image measurement has been carried out for optical systemsfor many applications. The basic concept of this idea is thatmeasurement of the output response function of a system for a specificinput can lead to characterization of error mechanisms. The approachthat is best utilized is one that could measure the spread function froma point or a line (commonly known as point spread function and linespread functions respectively). For a linear, locally-stationary system,the Fourier Transform of these functions will lead to a modulationtransfer function, which. This is challenging for partially coherentimaging but correlation approaches exist. Two difficulties arise withthis method of image assessment for optical lithography. First is theproblem with the separating of aberration types and understanding theircontribution to losses in the spread or transfer functions. Small levelsof aberration can have similar impact and identification of azimuthalorders will be difficult. The second set of challenges with this methodcomes with making the mask and detector artifacts that are small enoughto give the resolution required for images of interest, accuratelyproducing arrays of these features at the detector, and gettingsufficient energy though a small “pinhole” or slit feature. An approachto this technique has been described by workers a Bell Labs and U.C.Berkeley [E. L. Raab et al, Proc. SPIE 2197 (1994) 550].

Wavefront Estimation Through Masking and Illumination

Several workers have developed and demonstrated in-situ methods to inferlens aberration and wavefront shape through use of particular maskfeatures. One technique that has matured to a reasonable commerciallevel is the phase shift focus monitor test developed by IBM [T. Brunneret al, Proc. SPIE 2197 (1994) 541]. Through the use of techniquessimilar to those used with phase shift masking approaches, aberrationscan be estimated from image and focus shifts. This method of measurementleads to an estimation based on knowledge of how a particular aberrationshould influence a particular image. The phase shift focus monitorapproach is very useful for fitting low order aberration butdiscrimination over a given azimuthal term is difficult. It is expectedthat a good deal of work will continue in this area, allowing thelithographer to get a better understanding of the performance of alithography tool. Test methods can be developed to measure specificportions of a wavefront. Complete description of an aberrated wavefrontis difficult.

Other methods of pupil sampling can be used to measure particularportions of a wavefront. With the use of any resolution enhancementtechnique (RET) such as phase shift masking (PSM) or off-axisillumination (OAI), particular potions of a pupil are utilized, leadingto a more discrete sampling of a wavefront than would occur withconventional partially coherent illumination. This can be takenadvantage of by designing illumination or phase masking that resonateswith particular aberrations. As an example, an alternating phase shiftmask structure can be quite sensitive to astigmatism and 3-point. Theimages of such features are then measured and compared with simulatedimages using known levels of aberration. The accuracy of matching anaberrated wavefront using this type of estimation is increased byincluding a range of different conditions and by limiting evaluation tothose conditions that would most likely be experience in a real imagingsituation. A method of wavefront sampling using binary line maskstructures is also describe in EP0849638, where the amount of aberrationis determined on the basis of a difference between line widths. Thismethod is adequate for the detection of comatic aberration but it isdifficult to extract the magnitude of such aberrations or the presenceof other aberrations.

Hartmann and Other Screen Tests

Perforated screen methods were first devised to eliminate thesensitivities associated with interferometric methods used for wavefrontmeasurement, most specifically air turbulence. A good review iscontained in [I. Ghozeil, Optical Shop Testing: Hartmann and OtherScreen Tests, D. Malacara ed, (1992) 501]. The basic concept of a screentest is that a wavefront can be sampled at a number of locations acrossa pupil in a predetermined fashion, allowing for reconstruction byrelating these sampled points to one another. The use of a portion of awavefront creates a focus position that is not coincidental with theideal focus of an entire wavefront. A tilt term results, which can becalculated based on the geometry of the optic being tested. Using thisapproach, any tilt aberration in the lens can be measured as a deviationform this predicted result. Using a number of sampling points, wavefrontaberrations can be mapped. Sampling screens of various types have beendevised over the years. Hartmann first described a radial screen [J.Hartmann, Zt. Instrumentenkd., 24, 1 (1904)], which had been most commonuntil the square array screen tests suggested first by Shack andemployed by various workers. Radial screens have been used for testinglarge concave mirrors, especially for telescopes. The advantage of thesquare array is the removal of circular symmetry, and the assumptionsthat can lead to artifact circular error buildup. A much higher surfacesampling can also be obtained. Also, the fabrication and measurement ofa rigid square array screen can ensure accuracy of wavefront metrology.One problem screen type methods inherently possess is the inability todetect small scale surface changes taking place between the holes in thescreen. These tests are often combined with other techniques to improvecapability.

Additional challenges encountered with screen tests include methods ofdata collection and data reduction. The use of electro-optical detectorarrays has been described for data collection [E. T. Pearson, Proc. SPIE1236, 628 (1990)], which is commonly performed using photographicplates. Rapid data collection is permitted and averaging is permitted.An additional improvement with the use of an electro-optical detector isan interferometric capability that can be included by intentionallyoverlapping sampling spots. This can allow closer packing of samplingspots and can lead to higher accuracy across the pupil. An additionaladvantage of such a detector is the ability to filter low intensitynoise artifacts.

The Hartman test has been improved upon and has found its way intomicrolithographic applications. Through use of rigid screens withprecise control over placement and tilt, measurement of projection lenswavefront is possible. The application of Fourier transform methods ofdata analysis [describe for instance by F. Roddier, Soc. Photo-Opt.Eng., 1237, 70 (1990)] assists with automation and the handling of largeamounts of data. Canon has disclosed a variation to the Hartmann test[U.S. Pat. No. 4,641,962 (1987)] for measuring wavefront aberration of atest optic in a reverse projection scheme. This test technique is notdescribed for use in-situ in a projection system but is indicative ofthe developments that have been made with Hartmann type tests for modernlens metrology.

A method referred to as the Litel method ([U.S. Pat. Nos. 5,978,085 andU.S. 5,828,455) uses a reticle consisting of a multiplicity of smallopenings. The method is a variation of a square array Hartmann screentest, often referred to as a Shack-Hartmann screen test. Several reviewshave been published on this technology, [N. Farrar et al, Proc. SPIE4000 (2000)]. The advantage of placing the screen at the reticle planeis in the positional accuracy that can be obtained in currentmicrolithographic tools. Placing the screen at this position in theoptical train requires additional optical components to be incorporatedinto the imaging system, which are added to the reticle instrument. Afundamental problem with screen tests is the inability to test wavefrontpositions between those tested with the screen openings

Phase Contrast Tests

Zernike first proposed using an improvement to the Foucault test, whichhas become known as a phase contrast or phase modulation test [F.Zernike, Mon. Not. R. Astron. Soc., 94, 371 (1934)]. This technique (andothers also developed by many workers since) uses a phase shifted diskartifact in the optical path so that the resulting phase delay isrecorded and can be correlated to wavefront aberration. Wolter developeda λ/2 phase edge test, which is considered a variation of the knife edgeor wire test where the phase edge removes the need to use a physicalmethod to block light [H. Wolter, Handbook of Physics, Vol. 24,Springer-Verlag, Berlin (1956), 582]. This improvement has becomeinteresting for applications requiring in-situ measurement.

The most recent modification to a phase contrast testing method (similarto the Wolter test) is the DART (Dirkson Annular Ring Test) methoddeveloped by Dirkson [P. Dirkson et al, Proc. SPIE 3679 (1999) 77] anddescribed in U.S. Pat. Nos. 6,248,486 and U.S. 6,368,763. The DARTmethod employs a test object which comprises a single closed figurehaving a phase structure. The closed phase object is generally sized inthe reticle plane with diameter ˜λ/NA and a phase of λ/2. The image ofthis phase edge ring is printed into resist. The cross section of thering is a convolution of the point spread function of the imaging toolat the particular condition of illumination with the resist responsefunction. The image is scanned using a detection device such a scanningelectron microscope (SEM). The scanned image is then subjected toanalysis to ascertain lens aberration. The ring image allows forevaluation of wavefront aberration at various azimuthal (angular)positions. Calibration and correlation of this ring image to wavefrontaberration involves the deconvolving of the resist function and fittingalgorithms to extract primary and higher aberration terms.

The degree to which this type of method can estimate an aberratedwavefront depends on the portions of the lens pupil that are used tocreate the measured image. Maximum sensitivity will be obtained usingthis method at low sigma levels. As partial coherence is decreased,however, less of the full lens pupil is utilized to image the phase edgeand correlation to full wavefront information is difficult. It has beensuggested that sampling over a range of illumination conditions canimprove the estimation. This complicates the process to some degree byrequiring multiple exposure and measurement passes. The extraction andinterpretation of aberrations from the images is often difficult andtime consuming because of the often subtle shape deformation that isintroduced into the ring images with low and moderate levels ofaberration. Large amounts of data are often needed for conclusiveresults. Consequently, the method is often only practiced by individualsthat are well trained in the fitting and interpretation of the ringimage results.

SUMMARY OF THE INVENTION

An object of this invention is to provide a convenient method for thedetection of lens aberration that can be employed during the standardoperation of a projection system, that is through the exposure of aphotoresist coated substrate through illumination of a mask test targetusing a radiation source and an illumination apparatus. Furthermore, themethod of the invention allows for the detection of specific aberrationtypes and trends, as well as levels of aberration, though visualinspection of high resolution images of resist patterned as well asthrough the fitting of aberration parameters through the means ofmathematical analysis of images and fitting algorithms. The test methodcomprises a test target which comprises at least one open figure whichcomprises a multiple component array of phase zones, where the multiplezones are arranged within the open figure so that their response to lensaberration is interrelated and the zones respond uniquely to specificaberrations depending on their location within the figure. This is aunique and new method of detecting a variety of aberration typesincluding coma, spherical, astigmatism, and three-point through theexposure of a photoresist material placed in the image plane of thesystem and the evaluation of these images. The test method offers theadvantage over other methods because of the sensitivity to particularabberation types, the unique response of multiple zones of the testtarget to aberrations, and the ease with which aberrations can bedistinguished. An open figure of the test target refers to a figurehaving no single contour line to close the figure, providing no boundaryline between the figure and the surrounding area.

The method of lens aberration detection is based on the identificationof the deviations that occur between the images printed with the openfigure test target and images that would be produced in the absence ofaberration. This can be carried out for example through the use oflithography simulation, where simulated images can be produced withoutaberration and with various levels of lens aberration. Comparisons ofprinted resist images to simulated resist images are made while thevalues of the coefficients for primary Zernike aberrations are varied.

The interrelationship among the multiple phase geometry is unique tothis invention and allows detection of aberration using the open figuretest target that is not possible through the use of a target thatconsists of single closed figures, as described in U.S. Pat. No.6,248,486. Also, the detection of aberration that is made possiblethrough the test object of the present invention is not possible using atest object consisting of structures defined only in amplitude, asdescribed in Chen in Ser. No. US2002/0088951.

The method of this invention is rejected in U.S. Pat. No. 6,248,486, thedisclosure of which is incorporated herein by reference, and whichdescribes a closed single figure. The method of this invention is alsorejected in Ser. No. US2002/0088951 where a plurality of non-resolvableamplitude-only features is arranged as a test target and thecircumstances associated with the use of phase patterns is described asproblematic. It is proposed that the non-resolvable amplitude-onlyfeatures are used to approximate the imaging effects of the closedsingle figure of U.S. Pat. No. 6,248,486. The method of the presentinvention is not obvious based on the disclosures of prior art. Theresponse of the test object of the present invention to lens aberrationthat is interrelated where the zones respond uniquely to specificaberrations depending on their location within the figure cannot bedescribed, predicted, or ascertained by the previous disclosures.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a projection imaging system that would employan embodiment of the method of the invention.

FIG. 2 is an x-oriented open multiple phase bar test object and itsimage in a photoresist.

FIG. 3 is a y-oriented open multiple phase bar test object and its imagein a photoresist.

FIG. 4 is an open multiple phase bar test object oriented at 45 degreesand its image in a photoresist.

FIG. 5 is a multiple open phase box test object and its intensity image.

FIG. 6 is a multiple open phase circle test object and its intensityimage.

FIG. 7 is a open phase test target with pairs of phase zones oriented at0, 45, 90, and 180 degrees.

FIG. 8 shows variations of open phase test targets using circular andsquare shaped components.

FIG. 9 is a plot of the diffraction energy within an objective lenspupil for a test target having L1=200 nm, L2=200 nm, and L3=200 nm.

FIG. 10 is a plot of the diffraction energy within an objective lenspupil for a test target having L1=200 nm, L2=150 nm, and L3=200 nm.

FIG. 11 is a plot of the diffraction energy within an objective lenspupil for a test target having L1=200 nm, L2=150 nm, and L3=100 nm.

FIG. 12 is a plot of the diffraction energy within an objective lenspupil for a test target having L1=200 nm, L2=200 nm, and L3=100 nm.

FIG. 13 is a resist image of an open phase test target showing theeffects of negative astigmatism.

FIG. 14 is a resist image of an open phase test target showing theeffects of positive astigmatism.

FIG. 15 is a resist image of an open phase test target showing theeffects of positive 45 degree astigmatism.

FIG. 16 is a resist image of an open phase test target showing theeffects of negative 45 degree astigmatism.

FIG. 17 is a resist image of an open phase test target showing theeffects of coma.

FIG. 18 is a resist image of an open phase test target showing theeffects of spherical.

FIG. 19 is a resist image of an open phase test target showing theeffects of three-foil.

FIG. 20 is a resist image of an open phase test target showing theeffects of multiple aberrations resulting in a total wavefront OPD of0.035 waves RMS with 0.10 microns of defocus.

FIG. 21 is a resist image of an open phase test target showing theeffects of no aberration.

FIG. 22 is a resist image of a test target which has a central zoneintensity of zero.

DETAILED DESCRIPTION OF THE INVENTION

When imaging with a projection optical system, the aberrations in thelens pupil introduce deformation to a wavefront resulting in imagingerrors. FIG. 1 shows a schematic of a projection imaging system. Anillumination apparatus 21 illuminates a mask test object 22 which isimaged through an objective lens 23 onto a photosensitized substrate 24.If a test object is employed on a photomask as a phase pattern,specifically with a phase shifted from that of the surrounding area by180 degrees, the lens aberration will introduce imaging errorscharacteristic of the aberration type and the mask geometry. As anexample, FIG. 2 shows how three small phase lines (between 0.5 and 1.5lambda/NA) are printed into a photoresist. The images are a result oflithographic simulation using a Prolith vector model (Prolith Version7.0, KLA FINLE) with a wavelength of 157 nm, a numerical aperture (NA)of 0.85, a partial coherence value of 0.30, and a resist thickness of 80nm. The resulting image consists of six separate lines, each occurringat the phase transitions on the photomask. The differences between thesix lines are a result of the random aberration programmed into thesimulator at a level corresponding to a root mean square (RMS) of 0.03waves (a reasonable level for such a lithography system). FIG. 3 showsthe effects of rotating the test object by 90 degrees, resulting in adifferent and unique effect to the resulting six lines, which isindicative of the radial and angular dependence of aberrations withinthe lens. FIG. 4 shows the effects of rotating the test object by 45degrees where results are also unique. It is desirable to detectaberration effects at all orientations or directions simultaneously. Thetest phase objects of FIGS. 2 through 4 can be reduced from multipleline groupings to multiple phase box groupings, as shown in FIG. 5. Thisfigure shows how the image intensity through the center the three boxesof sizes L1 and L3 spaced distance L2 apart and oriented along a 90–270degree axis results in six dark regions, or lines, in a clear field,similar the effect for the three bar patterns in earlier figures. Theseparation of the three dark regions is determined by the L1, L2, and L3dimensions. Furthermore, the shape of the phase features within the testtarget need not be square. FIG. 6 shows how circular phase shapesproduce similar intensity profiles along a central region of a multiplepattern grouping.

A preferred embodiment of the invention is shown in FIG. 7. In thisembodiment, a test target is designed as a grouping of phase zones sothat the test target is a single open figure based on this collection ofzones. The test target is referred to as a Wheel Aberration Target. Thephase of the zones are shifted with respect to the phase of thesurrounding field by 180 degrees. The features are sized in rangesbetween 0.5 and 1.5 lambda/NA and the entire open phase test target isbetween 2.5 and 5 lambda/NA in size. The interrelationships between thegeometry is unique for each region within the test target as eachportion has a unique proximity to surrounding phase values. These uniqueproximity conditions for regions in the target of FIG. 7 are describedbelow.

Region of target Feature size Neighborhood Center L3 Features sized L1spaced L2 at 0, 45, 90, 135, 180, 225, 270, and 315 degrees Top L2Feature sized L3 spaced L2 below Feature sized L2 at 315 and 45 degreesBottom L2 Feature sized L3 spaced L2 above Feature sized L2 at 225 and135 degrees Right L2 Feature sized L3 spaced L2 to left Feature sized L2at 45 and 135 degrees Left L2 Feature sized L3 spaced L2 to rightFeature sized L2 at 315 and 225 degrees Top-Right L2 Feature sized L3spaced L2 below-left Feature sized L2 at 0 and 90 degrees Top-Left L2Feature sized L3 spaced L2 below-right Feature sized L2 at 0 and 270degrees Bottom-Right L2 Feature sized L3 spaced L2 above-left Featuresized L2 at 180 and 90 degrees Bottom-Left L2 Feature sized L3 spaced L2above-right Feature sized L2 at 180 and 270 degrees

These multiple zones of the test object form a single figure with noclosed form. This open phase nature of the test target is a unique andsignificant benefit of the invention. Unlike a closed figure having aphase structure that is designed so that all regions around thestructure are identical regardless of azimuthal position, each locationof the multiple feature open phase target of the invention is unique.Furthermore, the test structure of the present invention can take onforms different than that shown in FIG. 7. Examples of test targets withnine phase features are shown in FIG. 8. These targets consist ofgroupings of square and circular phase features of various sizing andshaping. The shape of the features within the target is less criticalthan the sizing and spacing since the detailed structure of the featuresis likely beyond the resolution of the lithography system. Thetransmission of the zones can be varied. In one embodiment, thetransmission of all zones and all surrounding regions is unity. Inanother embodiment of the invention, the center zone has a transmissionof zero. In each circumstance, the response of the zones within the testobject to lens aberration are interrelated and the zones responduniquely to specific aberrations depending on their location within thefigure. Also, the grouping of the features is not limited to thisdescriptive example. Variations in the number of features, shaping,sizing, phase, transmission, and density can be modified with the sameopen phase test target effect where the interrelationship among thephase features allows for the unique detection of aberrations.

Aberrations influence imaging through the deformation that they producein a wavefront within a lens pupil. Consequently, it is desirable todesign an aberration test target so that it will sample a lens pupil inthe most beneficial fashion. Since aberrations have unique character inthe manner which they influence specific portions of a lens pupil, thetest target of the present invention can be designed so that it is mostsensitive to particular aberration types and order. As an example, FIG.9 shows the magnitude of the diffraction energy within the objectivelens pupil for a test object comprising zones with sizing valuescorresponding to L3=200 nm, L2=200 nm, and L1=200 nm for a 157 nmwavelength imaging system operating at a numerical aperture of 0.85.FIG. 10 shows the magnitude of the diffraction in the objective lenspupil for a test object comprising zones with sizing valuescorresponding to L3=200 nm, L2=150 nm, and L1=200 nm for a 157 nmwavelength imaging system operating at a numerical aperture of 0.85.FIG. 11 shows the magnitude of the diffraction in the objective lenspupil for a test object comprising zones with sizing valuescorresponding to L3=200 nm, L2=150 nm, and L1=100 nm for a 157 nmwavelength imaging system operating at a numerical aperture of 0.85.FIG. 12 shows the magnitude of the diffraction in the objective lenspupil for a test object comprising zones with sizing valuescorresponding to L3=200 nm, L2=200 nm, and L1=100 nm for a 157 nmwavelength imaging system operating at a numerical aperture of 0.85. Thedistribution of the diffraction energy within a lens pupil is unique foreach example and shows how a test object can be designed for particularsensitivity to an aberration order. The diffraction energy distributionof FIG. 11 for example is most sensitive to 3^(rd) order (or primary)aberration and the sizing values of this example are used for theremaining examples of the present description.

FIGS. 13 through 21 show simulated resist images from a test object withone open figure which comprises a multiple component array of phasezones, where sizing dimensions correspond to L3=200 nm, L2=150 nm, andL1=100 nm for a wavelength of 157 nm and a numerical aperture of 0.85.The zones within the figure are circular. The phase of the circularregions are phase shifted from the surrounding region by 180 degrees.The transmission of the figure is unity.

FIG. 13 shows the unique impact of negative 3^(rd) order astigmatismthrough a defocus range of +/−0.12 microns. The unique behavior of theimages resulting from the method of the invention is the characteristicdeformation of the images printed from the zones within the test targetat X and Y orientations. In the presence of negative astigmatism,positive defocus causes the opening of the zones at the extreme Ylocations. In the presence of negative astigmatism, negative defocuscauses the opening of the zones at the extreme X locations.

FIG. 14 shows the unique impact of positive 3^(rd) order astigmatismthrough a defocus range of +/−0.12 microns. The unique behavior of theimages resulting from the method of the invention is the characteristicdeformation of the images printed from the zones within the test targetat X and Y orientations. In the presence of positive astigmatism,positive defocus causes the opening of the zones at the extreme Xlocations. In the presence of positive astigmatism, negative defocuscauses the opening of the zones at the extreme Y locations.

FIG. 15 shows the unique impact of positive ₃ ^(rd) order 45 degreeastigmatism through a defocus range of +/−0.12 microns. The uniquebehavior of the images resulting from the method of the invention is thecharacteristic deformation of the images printed from the zones withinthe test target at diagonal orientations. In the presence of positive 45degree astigmatism, positive defocus causes the opening of the zones atthe extreme −45 degree locations. Negative defocus causes the opening ofthe zones at the extreme +45 degree locations.

FIG. 16 shows the unique impact of negative 3^(rd) order 45 degreeastigmatism through a defocus range of +/−0.12 microns. The uniquebehavior of the images resulting from the method of the invention is thecharacteristic deformation of the images printed from the zones withinthe test target at diagonal orientations. In the presence of negative 45degree astigmatism, positive defocus causes the opening of the zones atthe extreme +45 degree locations. Negative defocus causes the opening ofthe zones at the extreme −45 degree locations.

FIG. 17 shows the unique impact of 3^(rd) order coma. The uniquebehavior of the images resulting from the method of the invention is thecharacteristic deformation of the images printed from the zones withinthe test target at all orientations. Coma aberration leads to thecharacteristic deformation of the images printed from the zones withinthe test target so that the zones are opened and oriented toward a pointcorresponding to the coma aberration. The outermost zone along thedirection of the coma aberration, and opposite in sign, remains closed.Vectors can be drawn from the openings within the zones, which directedopposite in sign along the direction of the coma aberration, convergingto a point at the edge of the target.

FIG. 18 shows the unique impact of spherical aberration. Sphericalaberration causes a distinct expansion and contraction of the zoneswithin the target. The effects are symmetrical within the target, whichis an indication of the symmetrical nature of spherical aberration. Thefigure shows the effect of defocus values of −0.16, 0.12, +0.12, and+0.16 microns of defocus for negative and positive aberration.

FIG. 19 shows the unique impact of 3^(rd) order 3-point aberration. Theunique behavior of the images resulting from the method of the inventionis the characteristic deformation of the images printed from the zoneswithin the test target at all orientations. 3-point aberration leads tothe characteristic deformation of the images printed from the zoneswithin the test target so that the zones are opened and oriented towarda point corresponding unique to the 3-point aberration. Vectors can bedrawn from the openings within the zones, which directed opposite insign along the direction of the 3-point aberration, converging to apoint within the target. Unlike the coma aberration effects, theinfluence of 3-point is a deformation of all zones and a convergence ofvectors corresponding to a 120 degree symmetry of the aberration.

FIG. 20 is a simulated resist image of an open phase test target showingthe effects of multiple aberrations. The effects of coma, astigmatism,spherical, and 3-point aberration combine to produce combineddeformation effects on the zones of the test target. The total wavefrontaberration in this example is 0.03 waves (RMS). The contribution fromprimary aberrations is: −0.0025 waves of astigmatism, 0.0091 waves of 45degree astigmatism, −0.0093 waves of x-coma, 0.0227 waves of y-coma,−0.0207 waves of spherical, −0.0676 waves of 3-point, and 0.0422 wavesof 45 degree 3-point.

FIG. 21 is a simulated resist image of an open phase test target showingthe effects of no aberration. The zones within the test target open to acentral region of the target.

FIG. 22 is a resist image of a test target which has a central zoneintensity of zero showing the effects of the multiple aberrations. Theeffects of coma, astigmatism, spherical, and 3-point aberration combineto produce combined deformation effects on the zones of the test targetsimilar to the previous examples where the central zone intensity isunity.

The test object of the present invention is achieved as a photomaskwhich is fabricated using methods that are common to phase-shiftphotomask fabrication. The steps involved in the fabrication of the maskinclude the layout of the test object using computer aided design ofother methods, exposing a sensitized polymer film coated over a quartzplate which may also have a masking film, developing the exposed image,and transferring the image into the photomask using a dry or wet patterntransfer process. The phase shift within the regions of the test objectare created through the etching of the quartz substrate to a depthcorresponding to 180 degrees, with possible depth correctionincorporated to account for phase effects of the relief structure of themask. Alternative methods can be employed with the same effect,including the deposition of layers to achieve phase definition.Transmission of the test object can be controlled through patterning ofthe masking layer.

It should be particularly noted that the reference (substantiallyaberration free) image is produced or created through lithographicmodeling and simulation. Aberrations are added to the simulation toallow for fitting by comparison of the simulated result to the resultimaged via lithography. An iterative process is carried out whereconvergence to the lens aberration is achieved by comparison of thesimulated result to the lithographic result. Alternatively, othermethods of fitting the simulated result to the lithographic result canbe used, such as, for example, mathematical fitting of shape parametersto the imaged target by polynomial fitting of curved edges, fittingparameters to target openings, fitting parameters to sizing and/orshifting results, to converge on an aberration level that would haveresulted in such pattern deformation.

It should also be particularly noted that the analysis of thelithographic test image is performed by using a magnifying device. Moreparticularly, the device is preferably scanned by a scanning detectiondevice, such as, for example, a scanning electron microscope. Thescanning detection device preferably coverts the scanned image intoimage data, which is then processed and displayed in a meaningful way,such as, for example, in graphs or diagrams, or is used to displayactual visual images of the observed structures on a display device,such as a monitor.

The present invention is a method to detect and measure aberrations inan optical system using a test target in the object plane of aprojection system and imaging a photoresist film with the system. Theinvention is described above but it is to be understood that it is notlimited to these descriptive examples. The numerical values, structures,sizes, orientations, position, placement, and the like may be changed toaccommodate specific imaging conditions. The design, optimization, andanalysis methods for the invention can be incorporated into alithographic simulator, a design layout tool, a computer program, orother analysis tools.

1. A method for detecting and measuring aberrations in an optical systemcomprising: providing a test target with at least one open figureincluding a multiple component array of phase zones, wherein themultiple phase zones are resolvable by the optical system and arearranged within the open figure so that their responses to lensaberrations are interrelated and the phase zones respond uniquely tospecific aberrations depending on their location within the figure;placing the test target in an object plane of a projection system;imaging a photoresist film with the projection system; and comparing theimage in the photoresist film to a reference image without aberrationsto detect aberrations in the optical system.
 2. The method of claim 1wherein the differences between the imaged photoresist and the referenceimage indicate the type and degree of aberration.
 3. The method of claim1 wherein the optical system comprises microelectronic photolithographicequipment for exposing a semiconductor wafer to a photomask carrying apattern for a microelectronic device.
 4. The method of claim 1 whereinsize of the phase zones and the spaces between the phase zones arebetween 0.5 λ/NA to 1.5 λ/NA where λ is the wavelength of the lightexposing the target and NA is the numerical aperture of the exposuresystem.
 5. The method of claim 1 wherein the size of the target isbetween 2.0 λ/NA to 6.0 λ/NA where λ is the wavelength of the lightexposing the target and NA is the numerical aperture of the exposuresystem.
 6. The method of claim 1 wherein the phase zones are 180 degreesout of phase with respect to the rest of the target.
 7. The method ofclaim 1 wherein the phase zones are etched into the surface of thetarget.
 8. The method of claim 1 wherein the phase zones comprise atleast two zones with one phase zone larger than the other phase zone. 9.The method of claim 1 wherein the phase zones comprise at least twozones of substantially the same size.
 10. The method of claim 1 whereinthe phase zones comprise a central phase zone and plurality ofcircumferential phase zones wherein the central phase zone is largerthan the circumferential phase zones.
 11. The method of claim 1 whereinthe phase zones comprise a central phase zone and plurality ofcircumferential phase zones wherein the central phase zone issubstantially the same size as the circumferential phase zones.
 12. Themethod of claim 1 wherein the phase zones comprise a central phase zoneand plurality of circumferential phase zones wherein the central phasezone is smaller than the circumferential phase zones.
 13. The method ofclaim 1 wherein each phase zone is circular, rectangular, elliptical, orhexagonal.
 14. The method of claim 1 wherein the target comprises acentral phase zone and eight circumferential phase zones equallyangularly spaced from each other for detecting astigmatism, coma,spherical aberration and three point aberration.
 15. The method of claim1 wherein the test target has at least two circumferential phase zonesspaced 180 degrees apart from each other for detecting positive ornegative lens aberration.
 16. The method of claim 15 wherein the testtarget has at least two more circumferential phase zones spaced 180apart from each other and 90 degrees from the first two circumferentialphase zones for detecting positive and negative lens aberration.
 17. Themethod of claim 15 wherein the test target has at least fourcircumferential phase zones located at 0, 90, 180, 270 degrees and twomore phase zones at 135 and 315 degrees or 45 and 225 degrees to detect45 degree astigmatism.
 18. The method of claim 15 wherein the testtarget has phase zones with similar or different shapes.
 19. The methodof claim 1 wherein the test target has phase zones with circular,rectangular, elliptical, pentagonal, triangular or hexagonal shapes. 20.The method of claim 1 wherein the test target has phase zones with thesame shape.
 21. The method of claim 1 wherein the test target has acentral phase zone with one shape and circumferential phase zones with adifferent shape.
 22. A method of detecting aberrations of an opticalimaging system, comprising the steps of: arranging a test object in theobject plane of the system; providing a photoresist layer in the imageplane of the system; imaging the test object by means of the system andan imaging beam; developing the photoresist layer, and detecting thedeveloped image by means of a scanning detection device having aresolution which is considerably larger than that of the imaging system,characterized in that use is made of a test object which comprises atleast one open figure having a phase structure which is withinresolution limits of the optical imaging system, wherein the image ofthis figure is compared to a reference image of known or no aberrationin order to determine the type and amount of aberration in the opticalimaging system.